1. Field of the Invention
This invention relates in general to a method and apparatus for calibrating disk drives, and more particularly, to a method and apparatus for adaptively calibrating disk drive performance based on real time performance monitoring.
2. Description of Related Art
Over the past several years, most hard drive manufacturers have migrated from purely analog recording technology to drives that use digital electronics. This new technology includes digital read channels which have at least doubled the amount of information that can be packed into an area on a hard drive platter. Further advances have produced even greater improvements in density. For example, read channels implementing partial-response maximum-likelihood digital signal processing allow greater data density by enabling the disk drive to filter out the noise associated with densely packed data. One such example is U.S. Pat. No. 5,220,466, issued to Coker et al., entitled "METHOD AND APPARATUS FOR DIGITAL FILTER CONTROL IN A PARTIAL RESPONSE MAXIMUM-LIKELIHOOD DISK DRIVE SYSTEM", assigned to the assignee of this application, and incorporated by reference herein, which discloses controlling a digital filter function included in a data path within gain and timing control loops of a PRML magnetic recording channel. Such improvements in density are necessary as desktop machines move to applications that employ such rich data types as voice and video.
One application which employs rich data types such as voice and video data is video-on-demand. Video on Demand (VoD) is an interactive multimedia system that works like cable television, except that the customer can select a movie from a large video database. Individual customers in an area are therefore able to watch different programs when they wish to, making the system a realization of the video rental shop brought into the home.
The customer can call on a range of services from video-on-demand applications. While watching movies on television, performing operations such as video selection, pause, rewinding, etc., can be selected as if using a video player. Typically, these commands are processed by the set-top-box and sent to the local server. The local server processes the request if possible. Otherwise, it relays the request to a video archive server.
Digital video/audio information can be compressed and stored on hard disk and advertised for users on the network. Multiple archive servers can simultaneously be running over the same network, depending on the bandwidth available. Implementing a cost-effective and efficient video server is one of the most demanding engineering hurdles to be overcome. Such a server should have the capacity to hold hundreds of Gigabytes (or perhaps Terabytes) of digital information and provide simultaneous access to several hundreds (or thousands) of subscribers in real-time, giving each one an appropriate bandwidth (usually on the order of 1.5-6 megabits per second (Mbps)).
The storage subsystem consists of control units, disk storage and an access mechanism. There are several techniques for increasing the performance by interleaving the digital data of a program on multiple disks. For example, striping involves interleaving portions of disk blocks on multiple disk drives. The aim here is to reduce the latency for block access by parallel reading of the complete block, while attaining high data reliability. Declustering, on the other hand, distributes blocks of files on several disks within the same disk drive, thus allowing parallel block access from the same file and increasing the rate of the video stream. The overall performance can also be enhanced by replicating files among the servers by considering the demand for a program, relative dispersion of subscribers and the access patterns (eg., time/day of peak access, average number of simultaneous viewers, etc.).
Nevertheless, the disk drives of these video servers require extensive calibration to meet the demands of the video-on-demand environment. However, since the interruption of the data stream to perform such functions is undesirable, normal calibration techniques present unique problems. For example, current files require periodic calibration due to thermal changes and other factors. This limit prevents the file from meeting the video-on-demand specification. Currently, video-on-demand users have to disable the periodic calibration function to meet the constant data stream demand of video-on-demand. Due to the lack of thermal calibration, however, file performance will be significantly degraded.
Further, current files have to be designed to function under extreme ends of environmental variations. Power supply and temperature changes are just two examples. To accommodate these changes, additional design compromises have to be made. Yet it is important that system performance is not interrupted or degraded by calibration operations.
It can be seen then that there is a need for a method and apparatus for adaptively calibrating disk drive performance based on real time performance monitoring.